Hello Theo, sorry for that, i thought you were talking about the helloCoco Magento theme.
About the slider of helloCoco wordpress, you have to set a “featured image” to the post, so it will display in the slider.
Adding Thumbnails To establish an image as a post’s featured image, click on the Set featured image link as shown on the screenshot, on the right nav editing menu of any post. A media upload window will pop up, from which you need to either upload a new image, or use an existing attached one. If you need to upload a new image, click on Select Files, to bring up a files browser, and upload it. After WordPress has finished uploading and crunching it, a panel will slide down with options regarding such image. Scroll down to the bottom of it, to find an Use as featured image link, which you need to click. Save all changes afterwards, and update post to see the chosen image under the Featured Image left nav menu panel.